1. Field of the Invention
The present invention relates to an optical modulator with a monitor for use in optical communications, and more particularly to a Mach-Zehnder interferometric optical modulator with a monitor which has two branched optical waveguides for causing light waves propagated therethrough to interfere with each other.
2. Description of the Related Art
Optical modulation principles are roughly classified into a direct modulation process wherein a laser diode as a light source is directly controlled to modulate a laser beam emitted thereby and an external or internal modulation process wherein a semiconductor laser beam is externally or internally modulated. The former modulation process is mainly used for low-rate optical communications at communication rates up to 10 Gbps and the latter modulation process is mainly used for high-rate optical communications at higher communication rate.
Optical modulators based on the external modulation principles include a Mach-Zehnder interferometric optical modulator. The Mach-Zehnder interferometric optical modulator is widely used as an external modulator particularly for ultra-high-rate optical communication systems because it can provide modulation characteristics which are stable against disturbance and have a good S/N ratio by canceling out in-phase noise components with the push-pull application of a drive voltage.
FIG. 1A of the accompanying drawings shows a general Mach-Zehnder interferometric optical modulator. As shown in FIG. 1A, the Mach-Zehnder interferometric optical modulator has optical waveguide 82 embedded in the surface of optical substrate 81 having an electro-optic effect. Optical waveguide 82 includes input optical waveguide 82a divided into two optical waveguides 82b, 82c by a Y-shaped divider and output optical waveguide 82d combined from optical waveguides 82b, 82c by a Y-shaped coupler. The Mach-Zehnder interferometric optical modulator also has optical buffer layer 89 and traveling-wave electrode 84 in a certain pattern which are disposed on optical waveguides 82b, 82c. 
A single linearly polarized light beam applied to input optical waveguide 82a is divided by the Y-shaped divider into equal light beams which travel respectively through optical waveguides 82b, 82c. At this time, an electric field generated by applying a voltage to traveling-wave electrode 84 from high-frequency power supply 87 is applied vertically to optical waveguides 82b, 82c in opposite directions, as shown in FIG. 1B of the accompanying drawings. Because of the electric field thus applied, the refractive indexes of optical waveguides 82b, 82c are changed by the electro-optic effect of optical substrate 81. The changes of the refractive indexes of optical waveguides 82b, 82c are equal in quantity, but opposite in sign. Therefore, the changes of the refractive indexes modulate the phases of the light beams in optical waveguides 82b, 82c in a push-pull manner. The light beams that are phase-modulated in optical waveguides 82b, 82c by xc2x1xcfx86/2, respectively, are combined by the Y-shaped coupler into a light beam that travels through output optical waveguide 82d, which outputs the light beam from its output end. The output light beam changes by cos2(xcfx86/2) with respect to the total phase modulation xcfx86. For example, when the light beams traveling through optical waveguides 82b, 82c are combined in phase with each other (xcfx86=2nxcfx80), the output light beam is of a maximum output, and when the light beams traveling through optical waveguides 82b, 82c are combined in opposite phase with each other (xcfx86=(2n+1)xcfx80), the output light beam is of a minimum output (n=1, 2, 3, . . . ).
For optical intensity modulation, it is preferable to set an initial operating point of the Mach-Zehnder interferometric optical modulator shown in FIG. 1A to an intermediate point (xcfx80/2 phase) between the maximum and minimum outputs. To set such an initial operating point, there has been proposed an optical modulator design which is similar to the optical modulator shown in FIG. 1A except that it also has, as shown in FIG. 2A of the accompanying drawings, DC power supply 85 and bias circuit 86 in addition to high-frequency power supply 87 so as to be able to adjust the initial operating point. With the proposed optical modulator, in addition to the modulation signal (AC) voltage which is a drive voltage, a DC voltage for setting a bias is applied to the traveling-wave electrode 84 to change the refractive indexes of the optical waveguides due to the electro-optic effect of the optical substrate for thereby shifting the phase. FIG. 2B of the accompanying drawings shows output characteristics of the optical modulator shown in FIG. 2A at the time the DC voltage is 0 V.
The optical modulator shown in FIG. 2A is, however, disadvantageous in that it is unable to maintain stable modulation characteristics over a long period of time owing to time-dependent changes (DC drift) in the operating point. The DC drift often occurs if the optical substrate is made of lithium niobate crystal, for example.
In view of the above drawback, it has been proposed to detect a portion of the output light beam of the optical modulator as a monitor light beam, and supply the monitor light beam through a feedback loop to correct the applied voltage depending on a change in the electric field due to the DC drift. One proposed optical modulator with a monitor, which is disclosed in Japanese patent No. 2738078, is illustrated in FIG. 3 of the accompanying drawings.
The optical modulator shown in FIG. 3 is substantially similar to that of the optical modulator shown in FIG. 2A except that it has a structure for extracting a portion of the output light beam of the optical modulator as a monitor light beam and supplying the monitor light beam through a feedback loop. Those parts of the optical modulator shown in FIG. 3 which are identical to those of the optical modulator shown in FIG. 2A are denoted by identical reference characters.
In FIG. 3, input signal power supply 90 comprises high-frequency power supply 87, DC power supply 85, and bias circuit 86 shown in FIG. 2A, and is arranged to be able to adjust the initial operating point with the DC bias. To input optical waveguide 82a, there is connected single-mode optical fiber 92 which guides a light beam emitted by semiconductor laser 91 into input optical waveguide 82a. Output optical waveguide 82d is connected to single-mode optical fiber 93 which is branched into single-mode optical fibers 95, 96 by fiber coupler 94. A modulated light beam, i.e., a signal light beam, output from output optical waveguide 82d is divided by fiber coupler 94 into light beams that travel respectively through single-mode optical fibers 95, 96, from which the light beams are output. The modulated light beams, i.e., signal light beams, output from single-mode optical fibers 95, 96 are detected by respective photodetectors 97, 98. Photodetector 97 is a photodetector that belongs to a party with which to communicate. The photodetector 98 supplies its output signal to signal processor/controller 99.
The modulated light beam output from single-mode optical fiber 93 is divided by fiber coupler 94 into a light beam that is detected by photodetector 97 and a light beam that is detected by photodetector 98. Based on the light beam detected by photodetector 98, signal processor/controller 99 detects a change in the operating point and controls input signal power supply 90 and sends the detected change to input signal power supply 90 via a feedback loop for thereby adjusting the DC bias in input signal power supply 90 so as to catch up to a change in the electric field due to a DC drift.
The publication referred to above also proposes an optical modulator capable of monitoring light radiated from the optical substrate. FIGS. 4 and 5 of the accompanying drawings show such a proposed optical modulator with a monitor.
The optical modulator shown in FIG. 4 is similar to the optical modulator shown in FIG. 3 except that it has a structure for extracting light radiated from the optical substrate as a monitor light beam and supplying the monitor light beam through a feedback loop, instead of the structure for extracting a portion of the output light beam of the optical modulator as a monitor light beam and supplying the monitor light beam through a feedback loop. Those parts of the optical modulator shown in FIG. 4 which are identical to those of the optical modulator shown in FIG. 3 are denoted by identical reference characters.
As shown in FIG. 5, the propagated light partly leaks from the region where optical waveguides 82b, 82c are coupled to output optical waveguide 82d by the Y-shaped coupler, and the leaked light beam is radiated as radiated light 100 from a side of optical substrate 81 near the end face of output optical waveguide 82d. The total optical power and phase of radiated light 100 are complementary to those of the modulated light beam, i.e., the signal light beam, output from output optical waveguide 82d. In the optical modulator shown in FIG. 4, radiated light 100 is used as monitor light.
As shown in FIG. 4, signal light optical fiber 101 is coupled to the end face of output optical waveguide 82d, and monitor light optical fiber 102 for extracting radiated light 100 as monitor light is coupled to the side of optical substrate 81 near the end face of output optical waveguide 82d. These optical fibers 101, 102 are fixed in position by holder 103.
With the optical modulator shown in FIG. 4, the modulated light beam, i.e., the signal light beam, is propagated through signal light optical fiber 101 and detected by photodetector 97, and radiated light 100 is propagated through the radiated light optical fiber 102 and detected by photodetector 98. Based on the light beam detected by photodetector 98, signal processor/controller 99 detects a change in the operating point and controls input signal power supply 90 and sends the detected change to input signal power supply 90 via a feedback loop for thereby adjusting the DC bias in input signal power supply 90 so as to catch up to a change in the electric field due to a DC drift. It is also possible to recognize a modulated state of the light during communications by monitoring the output signal from photodetector 98.
The conventional optical modulators shown in FIGS. 3 and 4 suffer the following problems:
In the optical modulator shown in FIG. 3, since a portion of the modulated light beam, i.e., the signal light beam, is divided by the fiber coupler and used as a monitor light beam, the power of the transmitted signal light beam is reduced by the power of the divided monitor light beam. Therefore, the distance over which the signal light beam can be transmitted from the optical modulator is shortened. In addition, the fiber coupler that is required prevents the optical modulator from being reduced in cost and size.
In the optical modulator shown in FIG. 4, the radiated light from the optical substrate is used as the monitor light. The radiated light is radiated from the optical waveguide into the optical substrate when the light beams traveling through the branched optical waveguides are combined in opposite phase with each other, i.e., extincted, by the Y-shaped coupler. Since the radiated light spreads as it travels, only a portion of the radiated light reaches the end of the optical substrate, and hence the power of the radiated light that reaches the end of the optical substrate is small. Generally, because there is a trade-off between the detection sensitivity (minimum detection power level) of a photodetector and the detection range (maximum response frequency) thereof, the photodetector needs to have a high sensitivity if the power of the radiated light is small. The photodetector with a high sensitivity is expensive, or the detection range of the photodetector is unduly limited.
When the radiated light is monitored, the state of the modulated light or the signal light cannot accurately be recognized, and the radiated light may act as a noise component to lower the quality of optical communications.
It is therefore an object of the present invention to provide an optical modulator with a monitor which is small in size and low in cost and is capable of achieving an appropriate initial operating point and accurately recognizing the state of modulated light, i.e., signal light.
According to a first aspect of the present invention, an optical modulator with a monitor has a 3-dB directional coupler by which branched optical waveguides and output optical waveguides are coupled to each other, and photodetector means for detecting light output from one of the output optical waveguides as monitor light.
According to a second aspect of the present invention, an optical modulator with a monitor has a 2-input, 2-output multimode interferometric optical waveguide by which branched optical waveguides and output optical waveguides are coupled to each other, and photodetector means for detecting light output from one of the output optical waveguides as monitor light.
With the above arrangement, it is possible to control the optical modulator so that its operating point is shifted in advance by xcfx80/2.
The optical power at the time the light output of the optical modulator is extincted, i.e., when the light output is turned off, can be extracted via one of the output optical waveguides. Therefore, the optical power can be detected almost in its entirety as monitor light. Light radiated into an optical substrate of the optical modulator is prevented from acting as a noise component to lower the quality of optical communications.
The two light outputs from the 3-dB directional coupler are in opposite phase with each other and have an equal power. The two light outputs from the 2-input, 2-output multimode interferometric optical waveguide are in phase with each other and have an equal power. By monitoring one of the light outputs, the state of the other light output can be detected well without the phase or extinction ratio being degraded. The photodetector for detecting the monitor light is not required to be highly sensitive or expensive, unlike conventional optical modulators.
If the optical modulator has an optical path converter, then the monitor light can be extracted from a side of an optical modulator device which is different from a side thereof from which modulated light or signal light is output. Consequently, the photodetector for detecting the monitor light is held out of interference with an optical fiber for propagating the modulated light.
In any of the above arrangements of the present invention, a portion of the modulated light or signal light is not used as the monitor light. As a result, the power of the modulated light or signal light that is transmitted from the optical modulator is not unduly lowered. The optical modulator does not need a fiber coupler which would otherwise make it difficult to reduce the size and cost of the optical modulator.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.